Pathogenic microbes, such as gram-positive cocci, can produce an array of secreted or cell surface-associated virulence factors, which are capable of interfering with the host immune responses. A number of such virulence factors are binding proteins, which contain one or more domains that bind to host immunoglobulins. Such Immunoglobulin binding virulence factors which bind to the heavy chain constant region of the immunoglobulin are referred to at Immunoglobulin Binding Proteins (IgBPs). A subset of Immunoglobulin Binding Proteins, which interact with the Fc region of immunoglobulins, are referred to as Fc Binding proteins (FcBPs). This non-immune binding of immunoglobulins by IgBPs involves regions of the immunoglobulin outside of the antigen-antibody combining site. Such non-immune binding of host immunoglobulins by microbial virulence factors is thought to subvert the host anti-microbial immune response. Functionally, this can occurs through immuno-shielding by coating of the bacteria with Fc bound antibody, by blocking immunoglobulin Fc-mediated effector functions such as complement activation or Fc-receptor mediated binding to effector cells, or by expression of superantigens which interact with immune cell surface receptors.
In the case of Staphylococcus aureus (S. aureus), a number of immunoglobulin binding proteins are expressed, including Protein A (SpA), Sbi, SSL7 and SSL10.
There is some evidence that it is possible to generate an antibody response to highly purified surface components of S. aureus such as capsular polysaccharide, the collagen-binding protein Cna and the fibrinogen-binding protein ClfA. This has led to the discovery and clinical testing of a number of antibodies based therapies directed against such S. aureus antigens.
Despite promising preclinical activity, clinical trials of such agents have been met with little success. Infants with very low birth weights (<1500 g; <32 weeks gestation) are at a particular risk for nosocomial bacterial infection, as they have not benefited from trans-placental transfer of maternal antibodies. Many of these infections are caused by S. aureus. Altastaph® is a vaccine-induced hyperimmune polyclonal antibody with specificity for S. aureus serotype 5 and 8, developed by Nabi Biopharmaceuticals (US20060153857 A1). In spite of reaching target serum antibody levels, no decrease in S. aureus infection rates was observed in treatment groups in two clinical trials (Rupp et al., 2007; DeJonge et al., 2007). A second anti-S. aureus human immune sera, INH-A21 (Veronate®) was prepared by first screening donors for high titres against MSCRAMM (microbial surface components recognizing adhesion matrix molecules), (Inhibitex—U.S. Pat. No. 6,692,739). Although Phase II trials appeared promising at the highest antibody dose, Phase III of the trial did not observe any effect of antibody treatment in reducing the frequency of S. aureus infection.
Additional antibodies or antibody derived molecules which have been under development include Aurograb, an antibody that targets the immunodominant ABC transporter in MRSA (Weems et al., 2006), which was designed to blocks the multi-drug efflux pump, allowing antibiotics to retain effectivity; tefibazumab (Aurexis®) (U.S. Pat. No. 6,979,446), which targets Clumping Factor A (ClfA) and pagibaximab (BYSX-A110, US20080019976 A1), a chimeric antibody which binds lipoteichoic acid (LTA) present in the membrane of gram-positive bacteria. Elusys Theraputics has also attempted to developed a bispecific heteropolymer antibody by cross-linking an antibody directed against SpA with a second antibody the recognizes the CR1 receptors (WO 2008/140487 A2).
Currently, none of the approaches described above have shown significant activity in clinical trials. The development of new antibody based agents which overcome microbial immune evasion for the treatment or prevention of microbial infections, including S. aureus, is an important goal that would be of great clinical benefit.